Semiconductor integrated circuit for driving light emitting element, light emitting element driving device, light emitting device, and vehicle

ABSTRACT

A semiconductor integrated circuit for driving a light emitting element forms at least part of a light emitting element driving device configured to change the number of light emitting elements that are lit among a plurality of light emitting elements connected in series. The semiconductor integrated circuit includes a variable resistor controller configured to increase the resistance value of a variable resistor connected in series with the plurality of light emitting elements immediately before, or at the same time as, the time point at which the number of light emitting elements that are lit is reduced.

TECHNICAL FIELD

The invention disclosed herein relates to a semiconductor integratedcircuit for driving a light emitting element. The invention disclosedherein also relates to a light emitting element driving device, and to alight emitting device and a vehicle employing such a light emittingelement driving device.

BACKGROUND ART

A headlight of a vehicle is configured to be able to switch between astate in which it serves as a passing headlight by emitting a low beamand a state in which it serves as a cruising headlight by emitting ahigh beam that reaches farther frontward than the low beam.

One example of a light emitting device used as a headlight for a vehicleis disclosed in Patent Document 1. The light emitting device (LEDlighting circuit) disclosed in Patent Document 1 has a plurality oflight emitting elements connected in series and is configured to, in thestate serving as a cruising headlight, light all the light emittingelements and, in the state serving as a passing headlight, short-circuitpart of the light emitting elements to light only the rest of the lightemitting elements.

LIST OF CITATIONS Patent Literature

-   Patent Document 1: JP-A-2.013-47047 (Paragraphs 0029 to 0033)

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In the light emitting device (LED lighting circuit) disclosed in PatentDocument 1, when switching takes place from the state serving as acruising headlight to the state serving as a passing headlight, theoutput voltage of the DC-DC converter for driving a light emittingelement drops. The drop in the output voltage causes the electric chargestored in the output capacitor in the DC-DC converter to be dischargedfrom the output capacitor. This momentarily causes an overcurrent in theoutput current of the DC-DC converter, and the overcurrent momentarilypasses through the LEDs that are not short-circuited. That is, whenswitching takes place from the state serving as a cruising headlight tothe state serving as a passing headlight, the LEDs that are notshort-circuited are damaged.

In Patent Document 1, when switching is performed from a state of a highbeam for traveling to a state of a low beam for an oncoming car, theLEDs are short-circuited in two steps so that the output voltage of theDC-DC converter drops in two steps so as to reduce the overcurrent.

However, the light emitting device (LED lighting circuit) disclosed inPatent Document 1 merely reduces the magnitude of an overcurrent andprovides no fundamental solution to an overcurrent. Thus, the value of apredetermined output voltage Va resulting from the first-state drop inthe output voltage and the value of a retention time t1 for which tomaintain the predetermined output voltage Va needs to be optimizedexperimentally by cut-and-try method so that an overcurrent can bereliably reduced to a desired value. The optimum values of Va and t1mentioned above vary with changes in the specifications of LED orvariations among individual LEDs; thus, with the light emitting device(LED lighting circuit) disclosed in Patent Document 1, it is difficultto guarantee that the overcurrent is reduced to a desired value.

Other than switching from a high beam to a low beam as described above,controlling a lamp on a vehicle may involve, for example, control forlighting a plurality of light emitting elements sequentially, controlfor distinguishing a plurality of light emitting elements sequentially,control for lighting a plurality of light emitting elements arranged ina matrix to display an animation, control for lighting a plurality oflight emitting elements as an ADB (adaptive driving beam), and the like,and any such control may cause a high current to pass through lightemitting elements due to a decreased number of light emitting elementsthat are lit.

Means for Solving the Problem

According to one aspect of what is disclosed herein, a semiconductorintegrated circuit for driving a light emitting element forms at leastpart of a light emitting element driving device configured to change thenumber of light emitting elements that are lit among a plurality oflight emitting elements connected in series. The semiconductorintegrated circuit includes a variable resistor controller configured toincrease the resistance value of a variable resistor connected in serieswith the plurality of light emitting elements immediately before, or atthe same time as, the time point at which the number of light emittingelements that are lit is reduced.

According to another aspect of what is disclosed herein, a lightemitting element driving device includes an adjuster configured tochange the number of light emitting elements that are lit among aplurality of light emitting elements connected in series and a variableresistor controller configured to increase the resistance value of avariable resistor connected in series with the plurality of lightemitting elements immediately before, or at the same time as, the timepoint at which the number of light emitting elements that are lit isreduced.

According to yet another aspect of what is disclosed herein, a lightemitting device includes the light emitting element driving deviceaccording to the above configuration and the plurality of light emittingelements.

According to still another aspect of what is disclosed herein, a vehicleincludes the light emitting device according to the above configuration.

Advantageous Effects of the Invention

With a semiconductor integrated circuit for driving a light emittingelement, a light emitting element driving device, a light emittingdevice, and a vehicle according to what is disclosed herein, it ispossible to prevent a high current from passing through light emittingelements when the number of light emitting elements that are lit isdecreased.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing one configuration example of a lightemitting device.

FIG. 2 is a timing chart showing the waveforms of the output voltage ofa DC-DC converter, the voltage fed to light emitting diodes, and the LEDcurrent.

FIG. 3 is a timing chart showing the waveforms, observed with neither adischarge circuit nor a clamping element provided, of the output voltageof the DC-DC converter, the voltage fed to the light emitting diodes,and the LED current.

FIG. 4 is a timing chart showing the state of the light emitting diodes.

FIG. 5 is a diagram showing one configuration example of an LEDswitching circuit.

FIG. 6 is an exterior view (front view) of a vehicle mounted with alight emitting device.

FIG. 7 is an exterior view (rear view) of the vehicle mounted with thelight emitting device.

FIG. 8 is an exterior view of an LED headlight module.

FIG. 9 is an exterior view of an LED blinker lamp module.

FIG. 10 is an exterior view of an LED rear lamp module.

FIG. 11 is a diagram showing a modified example of the light emittingdevice.

DESCRIPTION OF EMBODIMENTS

In the present description, a MOS field-effect transistor denotes afield-effect transistor in which the gate is structured to have at leastthree layers: a layer of an electrical conductor or of a semiconductorsuch as polysilicon with a low resistance value, an insulation layer,and a P-type, N-type or intrinsic semiconductor layer. That is, thestructure of the gate of a MOS field-effect transistor is not limited toa three-layer structure composed of a metal, an oxide, and asemiconductor.

<Light Emitting Device>

FIG. 1 is a diagram showing one configuration example of a lightemitting device. The light emitting device shown in FIG. 1 includes alight emitting element driving IC 100. The light emitting device shownin FIG. 1 includes light emitting diodes Z0 to Z7 as light emittingelements and a light emitting element driving device for driving thelight emitting elements. The light emitting element driving deviceincludes the light emitting element driving IC 100.

The light emitting device shown in FIG. 1 includes a coil L1, anN-channel MOS field-effect transistor (hereinafter called an NMOStransistor) M1 as a switching element, a Schottky harrier diode D1, andan output capacitor C1.

The coil L1, the NMOS transistor M1, the Schottky barrier diode D1, andthe output capacitor C1 constitute a DC-DC converter of a boost/buck(step-up/-down) type. The DC-DC converter converts a direct-currentinput voltage V1 to a direct-current output voltage V_(OUT).

The light emitting device shown in FIG. 1 includes sense resistors R1and R2, a P-channel MOS field-effect transistor (hereinafter referred toas a PMOS transistor) M2 as a variable resistor, light emitting diodesZ0 to Z7, a diode D2, capacitors C2 to C4, and resistors R3 to R4.

The light emitting element driving IC 100 is a semiconductor integratedcircuit device (what is called an LED driver IC) having integrated in ita constant voltage circuit 1, a communication interface 2, a controllogic circuit 3, an LED (light emitting diode) switching circuit 4,operational amplifiers 5 and 6, an adder 7, an error amplifier 8, anoscillator 9, a slope circuit 10, a comparator 11, a boosting drivecircuit 12, an operational amplifier 13, an adder 14, a dischargecircuit 15, a clamping element 16, and diodes D3 and D4. The lightemitting element driving IC 100 further has, for establishing electricalconnection with the outside, a terminal YIN, a terminal GE, a terminalPOND, a terminal IS, a terminal SNSP, a terminal SNSN, a terminal PGATE,a terminal BOOT, a terminal PSW, terminals CH8 to CH0, a terminal TX, aterminal RX, a terminal COMP, and a terminal RT.

The input voltage V_(IN) is connected to the terminal VIN, to oneterminal of the coil L1, to one terminal of the output capacitor C1, andto the cathode of the diode D2. The other terminal of the coil L1 isconnected to the drain of the NMOS transistor M1 and to the anode of theSchottky barrier diode D1. The source of the NMOS transistor M1 isconnected to one terminal of the sense resistor R1 and to the terminalIS. The other terminal of the sense resistor R1 is connected to a groundpotential and to the terminal PGND. The gate of the NMOS transistor M1is connected to the terminal GL. Through the switching operation by theNMOS transistor M1, a switching voltage V_(SW) with a rectangularwaveform appears at the other end of the coil L1. The switching voltageV_(SW) is fed to one terminal of the capacitor C2. The other terminal ofthe capacitor C2 is connected to the terminal BOOT.

The cathode of the Schottky barrier diode D1 is connected to the otherterminal of the output capacitor C1, to one terminal of the senseresistor R2, to the terminal SNSP, and to the terminal OUT. At the otherend of the output capacitor C1, the output voltage V_(OUT) appears. Theoutput voltage V_(OUT) results from the switching voltage V_(SW) beingsmoothed. The output voltage V_(OUT) is fed to one terminal of thecapacitor C3, The other terminal of the capacitor C3 is connected to theterminal PSW.

The other terminal of the sense resistor R2 is connected to the sourceof a PMOS transistor M2 and to the terminal SNSN. The gate of the PMOStransistor M2 is connected to the terminal PGATE. The drain of the PMOStransistor M2 is connected to the terminal CH8. An LED drive current LEDpasses through the sense resistor R2 and the PMOS transistor M2 and isfed to, of the light emitting diode Z0 to Z8, the ones that are lit.

The anode of the light emitting diode Zm is connected to the terminalCH(m+1), and the cathode of the light emitting diode Zm is connected tothe terminal CHm. Here, m is any integer of zero or more but seven orless. The anode of the diode D2 is connected to the terminal CH0. Whenthe terminal CH0 is short-circuited to ground, the terminals VIN and GNDare short-circuited together, and a high current passes from theterminal VIN. When at least one of the terminals CH1 to CH7 isshort-circuited to ground, the diode D2 prevents a current to pass via aparasitic diode in a switch within the LED switching circuit 4. Thediode D2 can be, for example, a Schottky barrier diode.

One terminal of the resistor R3 for setting a clock frequency isconnected to the terminal RT. The other terminal of the resistor R3 isconnected to the ground potential.

One terminal of the resistor R4 is connected to the terminal COMP. Theother terminal of the resistor R4 is connected to the ground potentialvia the capacitor C4 for phase compensation.

The constant voltage circuit 1 generates a constant voltage V_(REG)using the input voltage V_(IN) fed in via the terminal V_(IN) and feedsthe constant voltage V_(REG) to different circuit blocks in the lightemitting element driving IC 100.

The communication interface 2 receives a signal fed in from the outsidevia the terminal RX and transmits the signal output from the controllogic circuit 3 to the outside via the terminal TX. The communicationinterface can be, for example, a UART (universal asynchronousreceiver-transmitter) interface. The signal that the communicationinterface 2 receives can be, for example, an instruction signal thatindicates the lighting states of the light emitting diodes Z0 to Z7. Thesignal that the communication interface 2 transmits can be, for example,a fault indication signal that indicates a fault.

The control logic circuit 3 controls the LED switching circuit 4 basedon the signal received by the communication interface 2. The controllogic circuit 3 sets the voltage division ratio of a division voltageV_(DIV) obtained by dividing the input voltage V_(IN) and the value of asecond voltage V2 corresponding to a target current. What is called DCdimming is achieved by changing the value of the second voltage V2.

Under the control of the control logic circuit 3, the LED switchingcircuit 4 selects, of the light emitting diodes 70 to Z7, the ones thatare to be lit. That is, the control logic circuit 3 and the LEDswitching circuit 4 change the number of light emitting diodes Z0 to Z7that are lit. The LED switching circuit 4 turns off the switchesconnected in parallel with the light emitting diodes that are to be litand turns on the switches connected in parallel with the light emittingdiodes that are to be extinguished. The LED switching circuit 4 uses asa supply voltage the voltage fed in via the terminal PSW. The cathode ofthe diode D4 is connected to the terminal PSW, the anode of the diode D4and the cathode of the diode D3 are connected to the terminal BOOT, andthe constant voltage V_(REG) is fed to the anode of the diode D3.

The operational amplifier 5 outputs the lower of the division voltageVow and the second voltage V2. In this way, when the division voltageVow is lower than the second voltage V2, it is possible to lower the LEDcurrent I_(LED) in accordance with the fall in the input voltage V_(IN).The output voltage of the operational amplifier 5 is fed to theinverting input terminal of the error amplifier 8.

The non-inverting input terminal of the operational amplifier 6 isconnected to the terminal SNSP, and the inverting input terminal of theoperational amplifier 6 is connected to the terminal SNSN. Theoperational amplifier 6 outputs a voltage in accordance with the voltageacross the sense resistor R2.

The output voltage of the operational amplifier 6 is offset by the adder7 to be 0.166 V higher. The first voltage V1 generated by the adder 7 isfed to the non-inverting input terminal of the error amplifier 8 and tothe non-inverting input terminal of the operational amplifier 13. Thefirst voltage V1 is a voltage based on the output current of the DC-DCconverter described above, that is, the LED current I_(LED).

The error amplifier 3 generates an error voltage V_(ERR) in accordancewith the difference between the first voltage V1 and the output voltageof the operational amplifier 5.

The oscillator 9 generates a clock signal CK. The clock frequency of theclock signal CK is determined by the resistance value of the resistor R3connected to the terminal RT. The clock signal CK is fed to the slopecircuit 10 and to the boosting drive circuit 12.

The slope circuit 10 generates a slope voltage V_(SLP) with a triangularor sawtooth-shaped waveform using the clock signal CK. The gradient ofthe slope voltage V_(SLP) varies in accordance with the voltage fed tothe terminal IS, that is, the source voltage of the NMOS transistor M1.

The comparator 11 compares the error voltage V_(ERR) with slope voltageV_(SLP) and feeds the comparison result to the boosting drive circuit12.

Based on the clock signal CK and the output from the comparator 11, theboosting drive circuit 12 generates a gate signal for the NMOStransistor and feeds it to the gate of the NMOS transistor Mt via theterminal GL.

With the circuit configuration described above, unless the divisionvoltage V_(DIV) is lower than the second voltage V2, the DC-DC converterdescribed above is subjected to feedback control such that the LEDcurrent LED is kept close to the target current.

According to an instruction from the control logic circuit 3,immediately before a time point at which the number of light emittingdiodes Z0 to Z7 that are lit is reduced, the operational amplifier 13increases the resistance value of the PMOS transistor M2 connected inseries with the light emitting diodes Z0 to Z7. The operationalamplifier 13 uses as a supply voltage the output voltage V_(OUT) fed tothe terminal OUT.

Grasping in advance how to control the lighting states of the lightemitting diodes Z0 to Z7, the control logic circuit 3 can, immediatelybefore a time point at which the number of light emitting diodes Z0 toZ7 that are lit is reduced, instruct the operational amplifier 13 toincrease the resistance value of the PMOS transistor M2.

By increasing the resistance value of the PMOS transistor M2, it ispossible to suppress an increase in the LED current I_(LED) on theoccasion of a reduction in the number of light emitting diodes Z0 to Z7that are lit.

The light emitting element driving IC 100 is configured not so as tosense an increase in the LED current LED and increase the resistancevalue of the PMOS transistor M2 based on the sensing result, but so asto increase the resistance value of the PMOS transistor M2 before theLED current I_(LED) increases. Thus, the light emitting element drivingIC 100 can increase the resistance value of the PMOS transistor M2 morepromptly than the configuration that senses an increase in the LEDcurrent I_(LED) and increases the resistance value of the PMOStransistor M2 based on the sensing result. That is, the light emittingelement driving IC 100 can suppress an increase in the LED current LEDmore effectively than the configuration that senses an increase in theLED current km and increases the resistance value of the PMOS transistorM2 based on the sensing result.

FIG. 2 is a timing chart showing the waveforms of the output voltageV_(OUT) of the DC-DC converter, the voltage V_(CH8) fed to the lightemitting diode Z7, and the LED current I_(LED). FIG. 2 is a timing chartobserved as the number of light emitting diodes Z0 to Z7 that are lit isdecreased one by one. The broken lines in FIG. 2 show time points atwhich the control logic circuit 3 decreases the number of light emittingdiodes Z0 to Z7 that are lit.

Starting at a time point at which the control logic circuit 3 decreasesthe number of light emitting diodes Z0 to Z7 that are lit, a period isobserved in which the difference between the output voltage V_(OUT) andthe voltage V_(CH8) is larger. However, since the resistance value ofthe PMOS transistor M2 is increased in advance, the PMOS transistor M2limits the LED current I_(LED); this suppresses an increase in the LEDcurrent I_(LED) during the period with the large difference between theoutput voltage V_(OUT) and the voltage V_(CH8).

In the embodiment, the operational amplifier 13 keeps the resistancevalue of the PMOS transistor M2 increased during a period from 10 μsbefore the time point at which the number of light emitting diodes Z0 toZ7 that are lit is decreased up to several tens of microseconds afterthe time point at which the number of light emitting diodes Z0 to Z7that are lit is decreased compared to in any other periods (duringsteady operation). How much to increase the resistance value of the PMOStransistor M2 is determined based on the period during which the outputvoltage Your is decreasing (the slew rate). For example, the resistancevalue of the PMOS transistor M2 may be kept increased until the PMOStransistor M2 momentarily turns off or within a range where the PMOStransistor M2 stays on. During steady operation, the operationalamplifier 13 keeps the PMOS transistor M2 fully on so as to minimize thepower loss in the PMOS transistor M2.

The length of time immediately before the time point at which the numberof light emitting diodes Z0 to Z7 that are lit is reduced is not limitedto 10 μs; instead, it can be set, for example, within a range between 1μs to 100 μs. The resistance value of the PMOS transistor M2 may beincreased, instead of immediately before the time point at which thenumber of light emitting diodes Z0 to Z7 that are lit is reduced, at thesame time as the time point at which the number of light emitting diodesZ0 to Z7 that are lit is reduced

Although the length of time immediately after the time point at whichthe light emitting diodes Z0 to Z7 that are lit is reduced is notlimited to 10 Its, it is preferable to set it, for example, within arange between 1 to 100 pts. The set periods immediately before andimmediately after the time point at which the light emitting diodes Z0to Z7 that are lit is reduced may be equal as in the embodiment or maybe different unlike in the embodiment.

The second voltage V2 is offset by the adder 14 to be 0.2 V higher. Thesecond voltage V2 having been offset by the adder 14 is fed to theinverting input terminal of the operational amplifier 13.

The operational amplifier 13 controls the PMOS transistor M2 inaccordance with the difference between the first voltage V1 and thesecond voltage V2 having been offset by the adder 14. Thus, when the LEDcurrent LED is higher than a target current by a predetermined value ormore, the operational amplifier 13 can limit the LED current LED byincreasing the resistance value of the PMOS transistor M2. Thepredetermined value mentioned above can be adjusted, for example, bychanging the value of the offset voltage (0.2 V in the embodiment) inthe adder 14. Moreover, when the LED current is higher than the targetcurrent by the predetermined value or more, this fails out of the steadyoperation mentioned above.

Although, in the embodiment, the adder 14 offsets the second voltage V2,a configuration is also possible where, instead of or in addition to theadder 14 offsetting the second voltage V2, an adder for offsetting thefirst voltage V1 is provided and the first voltage V1 having been offsetis fed to the non-inverting input terminal of the operational amplifier13. Also in this modified example, when the LED current I_(LED) ishigher than a target current by a predetermined value or more, theoperational amplifier 13 can limit the LED current LED by increasing theresistance value of the PMOS transistor M2. In this modified example,the predetermined value can be adjusted, for example, by changing thevalue of the offset voltage applied to the first voltage V1, or, foranother example, by changing at least one of the value of the offsetvoltage applied to the first voltage V1 and the value of the offsetvoltage applied to the second voltage V2.

The discharge circuit 15 draws a current from the output terminal of theerror amplifier 8 when the operational amplifier 13 is keeping theresistance value of the PMOS transistor M2 increased compared to duringsteady operation. In this way, when the resistance value of the PMOStransistor M2 is increased compared to during steady operation, theerror voltage V_(ERR) falls; thus, the boosting drive circuit 12controls the switching of the NMOS transistor 111 such that the LEDcurrent I_(LED) falls. It is possible to suppress the power loss in thePMOS transistor M2 when the resistance value of the PMOS transistor M2is increased compared to during steady operation.

There is no particular limitation on the value of the current drawn bythe discharge circuit 15. For example, the value of the current drawn bythe discharge circuit 15 is set based on the control state of theterminal PGATE in this case, the discharge circuit 15 may include acurrent mirror circuit and draw a mirror current of the current passingthrough the terminal PGATE from the output terminal of the erroramplifier 8. Or, for another example, the discharge circuit 15 mayinclude a constant current circuit and draw a constant current from theoutput terminal of the error amplifier 8. For yet another example, thedischarge circuit 15 may include a variable current circuit and set thevalue of the output current of the variable current circuit based on thecircuit constant of an externally connected discrete component, a signalreceived by the communication interface 2, or the like so as to draw theoutput current of the variable current circuit from the output terminalof the error amplifier 8.

The clamping element 16 clamps the lower limit of the error voltageV_(ERR). For the clamping element 16, for example, a Zener diode or thelike can be used. If the error voltage V_(ERR) becomes too low as aresult of the drawing of the current by the discharge circuit 15, it maylead to an insufficient LED current LED on return to steady operation.Providing the clamping element 16 helps prevent the LED current. LEDfrom becoming insufficient on return to steady operation.

FIG. 3 is a timing chart showing the waveforms, observed when thedischarge circuit 15 and the clamping element 16 are not provided, ofthe Output voltage V_(OUT) of the DC-DC converter, the voltage V_(CH8)fed to the light emitting diode Z7, and the LED current I_(LED). FIG. 3is, like FIG. 2 , a timing chart observed as the number of lightemitting diodes Z0 to Z7 that are lit is decreased one by one. Thebroken lines in FIG. 3 , like those in FIG. 2 , show time points atwhich the control logic circuit 3 decreases the number of light emittingdiodes Z0 to Z7 that are lit.

In FIG. 3 , the period with a large difference between the outputvoltage V_(OUT) and the voltage Wits and a large power loss in the PMOStransistor M2 is longer compared to in FIG. 2 . Thus, it is preferableto provide the discharge circuit 15 as in the embodiment. It is furtherpreferable to provide the clamping element 16 as in the embodiment so asto prevent the LED current LED from becoming insufficient on return tosteady operation.

The control logic circuit 3 can perform PWM (pulse-width modulation)dimming. Specifically, when performing PWM dimming, the control logiccircuit 3 generates a PWM signal and, based on the PWM signal, turns onand off the switches connected in parallel with the light emittingdiodes that are to be lit, That is, when PWM dimming is performed, thelight emitting diodes that are lit are not lit all the time, but whetherthey are lit or extinguished is switched based on the PWM

The control logic circuit 3 sets the duty of the PWM signal based on,for example, a signal received by the communication interface 2.

It is preferable that the control logic circuit 3 and the LED switchingcircuit 4 have a mode (phase shift triode) in which the timings ofswitching from lighting to extinction are shifted among the lightemitting diodes Z0 to Z7. When all the light emitting diodes Z0 to Z7are lit in the phase shift mode by PWM dimming, the control logiccircuit 3 and the LED switching circuit 4 can light the light emittingdiodes Z0 to Z7 as in the timing chart shown in FIG. 4 .

The broken lines in FIG. 4 show time points at which any of the lightemitting diodes Z0 to Z7 switches from lighting to extinction. The solidblack parts in FIG. 4 show extinction periods, and the hollow parts inFIG. 4 show the lighting periods.

FIG. 5 is a diagram showing one configuration example of the LEDswitching circuit 4.

The LED switching circuit 4 includes switches SW0 to SW7. The switchesSW0 to SW7 are individually turned on and off by the control logiccircuit 3. One terminal of the switch SW0 is connected to the terminalCH0. The other terminal of the switch SW0 and one terminal of the switchSW1 are connected to the terminal CH1. The other terminal of the switchSW1 and one terminal of the switch SW2 are connected to the terminalCH2, The other terminal of the switch SW2 and one terminal of the switchSW3 are connected to the terminal CH3. The other terminal of the switchSW3 and one terminal of the switch SW4 are connected to the terminalCH4. The other terminal of the switch SW4 and one terminal of the switchSW5 are connected to the terminal CH5. The other terminal of the switchSW5 and one terminal of the switch SW6 are connected to the terminalCH6. The other terminal of the switch SW6 and one terminal of the switchSW7 are connected to the terminal CH7. The other terminal of the switchSW7 is connected to the terminal CH8.

The LED switching circuit 4 includes a comparator 41 for sensing aground fault at the terminal CH0. The comparator 41 is a hysteresiscomparator and compares a voltage fed to the terminal CH0 with athreshold voltage to output the result of the comparison. If the voltagefed to the terminal CH0 is equal to or higher than the thresholdvoltage, the output signal of the comparator 41 is at low level (a levelindicating a normal state) and, if the voltage fed to the terminal CH0is lower than the threshold voltage, the output signal of the comparator41 is at high level (a level indicating a ground fault at the terminalCH0). The threshold voltage used in the comparator 41 shifts between azeroth threshold voltage V_(TH0) and a first threshold voltage V_(TH1)in accordance with the level of the output signal of the comparator 41.The output signal of the comparator 41 is transmitted to the controllogic circuit 3.

The LED switching circuit 4 includes a comparator 42 for sensing aground fault at the terminal CH2. The comparator 42 is a hysteresiscomparator and compares a voltage fed to the terminal CH2 with athreshold voltage to output the result of the comparison. If the voltagefed to the terminal CH2 is equal to or higher than the thresholdvoltage, the output signal of the comparator 42 is at low level (a levelindicating a normal state) and, if the voltage fed to the terminal CH2is lower than the threshold voltage, the output signal of the comparator42 is at high level (a level indicating a ground fault at the terminalCH2). The threshold voltage used in the comparator 42 shifts between asecond threshold voltage V_(TH2) and a third threshold voltage V_(TH3)in accordance with the level of the output signal of the comparator 42.The output signal of the comparator 42 is transmitted to the controllogic circuit 3.

The LED switching circuit 4 includes a comparator 43 for sensing aground fault at the terminal CH4. The comparator 43 is a hysteresiscomparator and compares a voltage fed to the terminal CH4 with athreshold voltage to output the result of the comparison. If the voltagefed to the terminal CH4 is equal to or higher than the thresholdvoltage, the output signal of the comparator 43 is at low level (a levelindicating a normal state) and, if the voltage fed to the terminal CH4is lower than the threshold voltage, the output signal of the comparator43 is at high level (a level indicating a ground fault at the terminalCH4). The threshold voltage used in the comparator 43 shifts between afourth threshold voltage V_(TH4) and a fifth threshold voltage V_(TH5)in accordance with the level of the output signal of the comparator 43.The output signal of the comparator 43 is transmitted to the controllogic circuit 3.

The LED switching circuit 4 includes a comparator 44 for sensing aground fault at the terminal CH6. The comparator 44 is a hysteresiscomparator and compares a voltage fed to the terminal CH6 with athreshold voltage to output the result of the comparison. If the voltagefed to the terminal CH6 is equal to or higher than the thresholdvoltage, the output signal of the comparator 44 is at low level (a levelindicating a normal state) and, if the voltage fed to the terminal CH6is lower than the threshold voltage, the output signal of the comparator44 is at high level (a level indicating a ground fault at the terminalCH6). The threshold voltage used in the comparator 44 shifts between asixth threshold voltage V_(TH6) and a seventh threshold voltage V_(TH7)in accordance with the level of the output signal of the comparator 44.The output signal of the comparator 44 is transmitted to the controllogic circuit 3.

The LED switching circuit 4 includes a comparator 45 for sensing aground fault at the terminal CH8. The comparator 45 is a hysteresiscomparator and compares a voltage fed to the terminal CH8 with athreshold voltage to output the result of the comparison. If the voltagefed to the terminal CH8 is equal to or higher than the thresholdvoltage, the output signal of the comparator 45 is at low level (a levelindicating a normal state) and, if the voltage fed to the terminal CH8is lower than the threshold voltage, the output signal of the comparator45 is at high level (a level indicating a ground fault at the terminalCH8). The threshold voltage used in the comparator 45 shifts between aneighth threshold voltage V_(TH8) and a ninth threshold voltage V_(TH9)in accordance with the level of the output signal of the comparator 45.The output signal of the comparator 45 is transmitted to the controllogic circuit 3.

In the embodiment, the output signals of the comparators 41 to 45 aretransmitted to the control logic circuit 3; instead, the OR of theoutput signals of the comparators 41 to 45 may be transmitted to thecontrol logic circuit 3. If the OR of the output signals from thecomparators 41 to 45 is at high level, at least one of the terminalsCH0, C112, CH4, CH6, and CH8 is short-circuited to ground. In theembodiment, the comparators 41 to 45 constitute a ground fault detector;instead, for example, only the comparator 41 may be provided or,contrariwise, a comparator or the like for sensing a ground fault at theterminal CH1 may be added.

When a ground fault is detected by the ground fault detector describedabove, the control logic circuit 3 operates as a protector that suspendssupplying current to the light emitting diodes Z0 to Z7. For example,when a ground fault is detected by the ground fault detector, thecontrol logic circuit 3 controls the boosting drive circuit 12 so as tostop the operation of the DC-DC converter described above. In this way,when a ground fault occurs, it is possible to prevent a current fromcontinuing to pass through the PMOS transistor M2 and cause loss in thePMOS transistor M2.

The LED switching circuit 4 includes an open detection circuit DETm thatdetects an open fault in the light emitting diode Zm. The open detectioncircuit DETm detects an open fault in the light emitting diode Zm if,when the switch SWm is off, the voltage across the light emitting diodeZm is higher than a predetermined value (a value slightly higher thanthe forward voltage across the light emitting diode Zm). Here, m is anyinteger of zero or more but seven or less.

The results of detection by the open detection circuits DET0 to DET7 aretransmitted to the control logic circuit 3. When an open fault in alight emitting diode is detected by at least one of the open detectioncircuits DET0 to DET7, the control logic circuit 3 turns on, of theswitches SW0 to SW7, the switch connected in parallel with the lightemitting diode that has been detected as open to form a bypass routebypassing the light emitting diode that has been detected as open. Inthis way, even if part of the light emitting diodes Z0 to Z7 becomeopen, it is possible to prevent all the light emitting diodes Z0 to Z7from being extinguished.

However, the bypass route formed as a result of detection of an openfault may cause an overcurrent in the LED current I_(LED) due to thedecreased number of light emitting diodes Z0 to Z7 that are lit. Thus,at the same time as detection of an open fault by at least one of theopen detection circuits DET0 to DET7, the control logic circuit 3increases the resistance value of the PMOS transistor M2. In this way,it is possible to suppress an increase in the LED current I_(LED)resulting from detection of an open fault. From the perspective offorming a bypass route as soon as possible on detection of an openfault, in the embodiment, instead of a bypass route being formed afteran increase in the resistance value of the PMOS transistor M2, formationof a bypass route is started at the same time as an increase in theresistance value of the PMOS transistor M2 is started. However, as inthe circuit example shown in FIG. 1 , in a configuration where theoutput voltage V_(OUT) of the DC-DC converter described above takes timeto increase after part of the light emitting diodes Z0 to Z7 become opendue to slow response in the DC-DC converter, on detection of an openfault in part of the light emitting diodes Z0 to Z7, it is possible toincrease the resistance value of the PMOS transistor M2 first and thenform a bypass route.

<Applications>

The light emitting device described above can be suitably used, forexample, as shown in FIGS. 6 and 7 , as a headlight (including a highbeam, a low beam, a position lamp, a fog lamp, etc. as necessary) X11 ofa vehicle X10, a light source for a daytime running lamp (DRL) X12, atail lamp (including a position lamp, a backup lamp, etc. as necessary)X13, a brake lamp X14, or a blinker lamp X15.

The light emitting devices described above may be provided as a module(such as an LED headlight module Y10 in FIG. 8 , an LED blinker lampmodule Y20 in FIG. 9 , and an LED rear lamp module Y30 in FIG. 10 ). Thelight emitting device may be provided in the form of a driving devicehaving a function of controlling the number of light emitting elementsto be lit, as a semi-product with the light emitting diodes, theexternally fitted components of the light emitting element driving IC,and the like omitted from the light emitting device described above.

<Others>

The embodiment disclosed herein should be considered to be in everyaspect illustrative and not restrictive, and the technical scope of thepresent invention is defined not by the description of embodiments givenabove but by the scope of the appended claims and should be understoodto encompass any modifications within a sense and scope equivalent tothe claims.

While the above embodiment deals with, as an example, a configurationusing light emitting diodes as light emitting elements, this is notmeant to limit how the present invention is to be implemented. Forexample, it is possible to use organic EL (electroluminescence) elementsas light emitting elements.

While the above embodiment deals with, as an example, a configurationusing PMOS transistors as variable resistors, this is not meant to limithow the present invention is to be implemented. For example, it is alsopossible to use any active elements other than PMOS transistors.

While, in the above embodiment, a single light emitting diode isconnected between the terminals CH(m+1) and CHm, a plurality of lightemitting diodes may be connected between the terminals CH(k+1) and CHk(k is any is any integer of zero or more but seven or less). When aplurality of light emitting diodes are connected between the terminalsCH(k+1) and CHk, they are handled as a group so that switching betweenlighting and extinction is performed for each of such groups. Also, itis necessary to control the voltage VCH8 fed to the terminal CH8 so thatit will not exceed a rated value.

FIG. 11 shows an example of a configuration where a plurality of lightemitting diodes are connected between the terminals CH(k+1) and CHk. Inthe modified example shown in FIG. 11 , two light emitting diodes areconnected between the terminals CH(k+1) and CM, where k=0, 1, 2, and 3.

While, in the embodiment described above, nine terminals CH0 to CH8 canbe connected to the light emitting diodes, the number of terminals thatcan be connected to the light emitting diodes is not limited to nine; itmay be any number of two or more other than nine.

While the embodiment described above deals with a configuration where alight emitting device includes a single light emitting element drivingIC 100, it is also possible to configure a part corresponding to thelight emitting element driving IC 100 with a plurality of ICs. Forexample, a part corresponding to the communication interface 2, thecontrol logic circuit 3, and the LED switching circuit 4 may beconfigured as an IC independent from the light emitting element drivingIC 100.

According to one aspect of what is disclosed herein, a semiconductorintegrated circuit for driving a light emitting element forms at leastpart of a light emitting element driving device configured to change thenumber of light emitting elements that are lit among a plurality oflight emitting elements connected in series. The semiconductorintegrated circuit includes a variable resistor controller configured toincrease the resistance value of a variable resistor connected in serieswith the plurality of light emitting elements immediately before, or atthe same time as, the time point at which the number of light emittingelements that are lit is reduced (a first configuration).

In the semiconductor integrated circuit according to the firstconfiguration described above, preferably, the light emitting elementdriving device includes a DC-DC converter that converts an input voltageto an output voltage and a switching controller that controls aswitching element in the DC-DC converter based on an error voltageoutput from an error amplifier that amplifies the difference between afirst voltage based on an output current of the DC-DC converter and asecond voltage corresponding to a target current. The light emittingelement driving device may be configured to feed the output current ofthe DC-DC converter to, of the plurality of light emitting elements, thelight emitting elements that are lit. The semiconductor integratedcircuit may include a discharger configured to draw a current from theoutput terminal of the error amplifier when the variable resistorcontroller is keeping the resistance value of the variable resistorincreased compared to during steady operation (a second configuration).

The semiconductor integrated circuit according to the secondconfiguration described above, preferably, further includes a clampingelement that clamps the lower limit of the error voltage (a thirdconfiguration).

The semiconductor integrated circuit according to any of the first tothird configurations described above, preferably, further includes anoffset processor configured to offset at least one of the first andsecond voltages. The variable resistor controller may be configured toadjust the resistance value of the variable resistor based on thedifference between the first and second voltages with the at least oneof the first and second voltages offset by the offset processor (afourth configuration).

The semiconductor integrated circuit according to any of the first tofourth configurations described above, preferably, further includes anadjuster configured to change the number of light emitting elements thatare lit. The adjuster may be configured to have a mode in which thetimings of switching from lighting to extinction are shifted among theplurality of light emitting elements (a fifth configuration).

The semiconductor integrated circuit according to any of the first tofifth configurations described above, preferably, further includes aground fault detector configured to detect a ground fault at least atone point in the series circuit of the plurality of light emittingelements and a protector configured to, when a ground fault is detectedby the ground fault detector, suspend supplying current to the pluralityof light emitting elements (a sixth configuration).

The semiconductor integrated circuit according to any of the first tosixth configurations described above, preferably, further includes anopen detector configured to detect an open fault for each of a pluralityof groups of light emitting elements among the plurality of lightemitting elements and a bypass part configured to form a bypass routethat bypasses, of the plurality of light emitting elements, the lightemitting element in which the open fault has been detected by the opendetector. The variable resistor controller may be configured to increasethe resistance value of the variable resistor at the same time as theopen fault is detected by the open detector (a seventh configuration).

According to another aspect of what is disclosed herein, a lightemitting element driving device includes an adjuster configured tochange the number of light emitting elements that are lit among aplurality of light emitting elements connected in series and a variableresistor controller configured to increase the resistance value of avariable resistor connected in series with the plurality of lightemitting elements immediately before, or at the same time as, the timepoint at which the number of light emitting elements that are lit isreduced (an eighth configuration).

According to yet another aspect of what is disclosed herein, a lightemitting device includes the light emitting element driving deviceaccording to the eighth configuration described above and the pluralityof light emitting elements (a ninth configuration).

According to still another aspect of what is disclosed herein, a vehicleincludes the light emitting device according to the ninth configurationdescribed above (a tenth configuration).

LIST OF REFERENCE SIGNS

-   -   constant voltage circuit    -   2 communication interface    -   control logic circuit    -   4 LED switching circuit    -   5, 6, 13 operational amplifier    -   7, 14 adder    -   8 error amplifier    -   9 oscillator    -   10 slope circuit.    -   11 comparator    -   12 boosting drive circuit    -   15 discharge circuit    -   16 clamping element    -   100 light emitting element driving IC    -   BOOT, CH0 to CH8, COMP, GL, IS, PGATE, POND, PSW, RT, RX, SNSN,        SNSP, TX, VIN terminal    -   C1 output capacitor    -   C2 to C4 capacitor    -   L1 coil    -   M1 NMOS transistor    -   M2 PMOS transistor    -   100 light emitting element driving IC    -   SW0 to SW7 switch    -   X10 vehicle    -   X11 headlight    -   X12 daytime running lamp    -   X13 tail lamp    -   X14 break lamp    -   X15 blinker lamp    -   Z0 to Z7 light emitting diode

1. A semiconductor integrated circuit for driving a light emittingelement, the semiconductor integrated circuit forming at least part of alight emitting element driving device configured to change a number oflight emitting elements that are lit among a plurality of light emittingelements connected in series, the semiconductor integrated circuitcomprising a variable resistor controller configured to increase aresistance value of a variable resistor connected in series with theplurality of light emitting elements immediately before, or at the sametime as, a time point at which the number of light emitting elementsthat are lit is reduced.
 2. The semiconductor integrated circuitaccording to claim 1, wherein the light emitting element driving deviceincludes; a DC-DC converter that converts an input voltage to an outputvoltage and a switching controller that controls a switching element inthe DC-DC converter based on an error voltage output from an erroramplifier that amplifies a difference between a first voltage based onan output current of the DC-DC converter and a second voltagecorresponding to a target current, the light emitting element drivingdevice is configured to feed the output current of the DC-DC converterto, of the plurality of light emitting elements, the light emittingelements that are lit, and the semiconductor integrated circuit includesa discharger configured to draw a current from an output terminal of theerror amplifier when the variable resistor controller is keeping theresistance value of the variable resistor increased compared to duringsteady operation.
 3. The semiconductor integrated circuit according toclaim 2, further comprising a clamping element that clamps a lower limitof the error voltage.
 4. The semiconductor integrated circuit accordingto claim 2, further comprising an offset processor configured to offsetat least one of the first and second voltages, wherein the variableresistor controller is configured to adjust the resistance value of thevariable resistor based on a difference between the first and secondvoltages with the at least one of the first and second voltages offsetby the offset processor.
 5. The semiconductor integrated circuitaccording to claim 1, further comprising an adjuster configured tochange the number of light emitting elements that are lit, wherein theadjuster is configured to have a mode in which timings of switching fromlighting to extinction are shifted among the plurality of light emittingelements.
 6. The semiconductor integrated circuit according to claim 1,further comprising: a ground fault detector configured to detect aground fault at least at one point in a series circuit of the pluralityof light emitting elements; and a protector configured to, when a groundfault is detected by the ground fault detector, suspend supplyingcurrent to the plurality of light emitting elements.
 7. Thesemiconductor integrated circuit according to claim 1, furthercomprising: an open detector configured to detect an open fault for eachof a plurality of groups of light emitting elements among the pluralityof light emitting elements; and a bypass part configured to form abypass route that bypasses, of the plurality of light emitting elements,the light emitting element in which the open fault has been detected bythe open detector, wherein the variable resistor controller isconfigured to increase the resistance value of the variable resistor atthe same time as the open fault is detected by the open detector.
 8. Alight emitting element driving device, comprising: an adjusterconfigured to change a number of light emitting elements that are litamong a plurality of light emitting elements connected in series; and avariable resistor controller configured to increase a resistance valueof a variable resistor connected in series with the plurality of lightemitting elements immediately before, or at the same time as, a timepoint at which the number of light emitting elements that are lit isreduced.
 9. A light emitting device comprising: the light emittingelement driving device according to claim 8, and the plurality of lightemitting elements.
 10. A vehicle comprising the light emitting deviceaccording to claim 9.